Reply by Joerg February 23, 20152015-02-23
On 2015-02-22 10:56 PM, Piotr Wyderski wrote:
> Joerg wrote: > >> Those are nice. Just keep in mind that they have a lowish UVLO. > > They come in 3 UVLO flavors. "B" is ~9.5V and "C" is 12V. >
Good. There is a surprising number of engineers these day who take drive levels a bit lightly yet use standard-level FETs. Even manufacturers of IC aren't paying enough attention to that. Until one fine day ... tsssk ... PHUT ... *KABLAM*
> > That extra supply isn't nice but as John Wayne said, "Man has got to do > > what man has got to do". It doesn't have to be regulated so even a > > simple gate drive transformer could be pressed into service to make an > > isolated supply. > > That's exactly how my test circuit works now. :-) > The quickly-wound transformer sucks in terms of leakage > inductance, but it can be improved. >
That's why I wind most of mine on toroids. As RF engineers say, toroids don't talk. At least not much if they are of the ferrite kind.
> Will brag about the results. ;-) >
Please do so. It can help others getting out of a similar pickle, years down the road. I once explained in detail how to change the bulb in an Italian office desk lamp. The designer kind where this is difficult. Many years later I kept receiving emails from people in a similar situation because the manufacturer link I had posted back then had gone limp. -- Regards, Joerg http://www.analogconsultants.com/
Reply by Piotr Wyderski February 23, 20152015-02-23
Joerg wrote:

> Those are nice. Just keep in mind that they have a lowish UVLO.
They come in 3 UVLO flavors. "B" is ~9.5V and "C" is 12V. > That extra supply isn't nice but as John Wayne said, "Man has got to do > what man has got to do". It doesn't have to be regulated so even a > simple gate drive transformer could be pressed into service to make an > isolated supply. That's exactly how my test circuit works now. :-) The quickly-wound transformer sucks in terms of leakage inductance, but it can be improved. Will brag about the results. ;-) Best regards, Piotr
Reply by Joerg February 20, 20152015-02-20
On 2015-02-20 12:06 PM, John Larkin wrote:
> On Fri, 20 Feb 2015 11:19:45 -0800, Joerg <news@analogconsultants.com> > wrote: > >> On 2015-02-19 7:36 AM, Winfield Hill wrote: >>> Piotr Wyderski wrote... >>>> >>>> Joerg wrote: >>>> >>>>> We know it's 17V and 50A but we'd also have to know >>>>> which FET you are using. That info is (hopefully) >>>>> in the datasheet :-) >>>> >>>> I haven't bought the FETs yet, but was thinking about >>>> IRFB7430PBF in TO220 or something bulkier in TO247. >>>> >>>> I naively thought that the 1524A max pulsed drain current >>>> is enough to survive even a short-circuit if disconnected >>>> in the ms range... >>>> >>>> Best regards, Piotr >>> >>> Whoa, it looks like there's a misunderstanding here. >>> If you short a strong low-resistance voltage source, >>> you actually want less current flowing, not more. >>> >>> Let's do the math. The 1524A rating of the '7430 is >>> for the case of a very low voltage across the MOSFET, >>> fully turned on, with only a volt or two of drop. >>> In fact, as the die rapidly heats, it'll current >>> limit at around 1kA, Fig 4, assuming 10V gate drive. >>> Trying to short out 17V, it would dissipate 17 kW. >>> >>> Z_thJC = dT_J(max) / Pd = 150/17kW = 0.009 &deg;C/W. >>> >>> Fig 14, Effective Transient Thermal Impedance, says >>> maximum allowed square single-pulse duration = 20us. >>> >>> That's the thermal mass calculation for the '7430. >>> >>> Let's try a higher voltage rated MOSFET (large-die >>> HV parts will limit at lower currents), say the >>> original IRF1407 version, an IRF1407PbF. It has an >>> impressive 333W dissipation rating, nearly as good >>> as the 375W rating of the '7430. But its 175&deg;C >>> current limit will be about 400A, or 6.8 kW. We >>> get Z_thJC = 150/7kW = 0.021 &deg;C/W, and datasheet >>> Fig 11 tells us the single-pulse duration = 35us. >>> So the "weaker" part can handle a full short for >>> almost twice as long. >>> >> >> With these little TO-packages it is more likely that there would be a >> bright orange flash, followed by a loud bang, plastic pieces shooting >> off in all directions, and an "amperage stench" :-) > > Where are the sound effects? I want my sound effects! > >
Phhsssseee ... tic .. tic-tic ... tic-tic-tic-tic ... PHOOF ... *BAM* No kidding, happened a few days ago. A class-D driver decided to go berserk while endurance-testing a circuit. I could get over there fast enough to turn it off. It took a lot of other stuff with it. All the fancy over-current protection in there didn't manage to protect the stage from death-spiraling. Replacing ICs on 2oz copper is a bear. This time the stench wasn't too bad though. Just a wee electric scent. -- Regards, Joerg http://www.analogconsultants.com/
Reply by John Larkin February 20, 20152015-02-20
On Fri, 20 Feb 2015 11:19:45 -0800, Joerg <news@analogconsultants.com>
wrote:

>On 2015-02-19 7:36 AM, Winfield Hill wrote: >> Piotr Wyderski wrote... >>> >>> Joerg wrote: >>> >>>> We know it's 17V and 50A but we'd also have to know >>>> which FET you are using. That info is (hopefully) >>>> in the datasheet :-) >>> >>> I haven't bought the FETs yet, but was thinking about >>> IRFB7430PBF in TO220 or something bulkier in TO247. >>> >>> I naively thought that the 1524A max pulsed drain current >>> is enough to survive even a short-circuit if disconnected >>> in the ms range... >>> >>> Best regards, Piotr >> >> Whoa, it looks like there's a misunderstanding here. >> If you short a strong low-resistance voltage source, >> you actually want less current flowing, not more. >> >> Let's do the math. The 1524A rating of the '7430 is >> for the case of a very low voltage across the MOSFET, >> fully turned on, with only a volt or two of drop. >> In fact, as the die rapidly heats, it'll current >> limit at around 1kA, Fig 4, assuming 10V gate drive. >> Trying to short out 17V, it would dissipate 17 kW. >> >> Z_thJC = dT_J(max) / Pd = 150/17kW = 0.009 &deg;C/W. >> >> Fig 14, Effective Transient Thermal Impedance, says >> maximum allowed square single-pulse duration = 20us. >> >> That's the thermal mass calculation for the '7430. >> >> Let's try a higher voltage rated MOSFET (large-die >> HV parts will limit at lower currents), say the >> original IRF1407 version, an IRF1407PbF. It has an >> impressive 333W dissipation rating, nearly as good >> as the 375W rating of the '7430. But its 175&deg;C >> current limit will be about 400A, or 6.8 kW. We >> get Z_thJC = 150/7kW = 0.021 &deg;C/W, and datasheet >> Fig 11 tells us the single-pulse duration = 35us. >> So the "weaker" part can handle a full short for >> almost twice as long. >> > >With these little TO-packages it is more likely that there would be a >bright orange flash, followed by a loud bang, plastic pieces shooting >off in all directions, and an "amperage stench" :-)
Where are the sound effects? I want my sound effects! -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by Joerg February 20, 20152015-02-20
On 2015-02-19 7:36 AM, Winfield Hill wrote:
> Piotr Wyderski wrote... >> >> Joerg wrote: >> >>> We know it's 17V and 50A but we'd also have to know >>> which FET you are using. That info is (hopefully) >>> in the datasheet :-) >> >> I haven't bought the FETs yet, but was thinking about >> IRFB7430PBF in TO220 or something bulkier in TO247. >> >> I naively thought that the 1524A max pulsed drain current >> is enough to survive even a short-circuit if disconnected >> in the ms range... >> >> Best regards, Piotr > > Whoa, it looks like there's a misunderstanding here. > If you short a strong low-resistance voltage source, > you actually want less current flowing, not more. > > Let's do the math. The 1524A rating of the '7430 is > for the case of a very low voltage across the MOSFET, > fully turned on, with only a volt or two of drop. > In fact, as the die rapidly heats, it'll current > limit at around 1kA, Fig 4, assuming 10V gate drive. > Trying to short out 17V, it would dissipate 17 kW. > > Z_thJC = dT_J(max) / Pd = 150/17kW = 0.009 &deg;C/W. > > Fig 14, Effective Transient Thermal Impedance, says > maximum allowed square single-pulse duration = 20us. > > That's the thermal mass calculation for the '7430. > > Let's try a higher voltage rated MOSFET (large-die > HV parts will limit at lower currents), say the > original IRF1407 version, an IRF1407PbF. It has an > impressive 333W dissipation rating, nearly as good > as the 375W rating of the '7430. But its 175&deg;C > current limit will be about 400A, or 6.8 kW. We > get Z_thJC = 150/7kW = 0.021 &deg;C/W, and datasheet > Fig 11 tells us the single-pulse duration = 35us. > So the "weaker" part can handle a full short for > almost twice as long. >
With these little TO-packages it is more likely that there would be a bright orange flash, followed by a loud bang, plastic pieces shooting off in all directions, and an "amperage stench" :-) -- Regards, Joerg http://www.analogconsultants.com/
Reply by Joerg February 20, 20152015-02-20
On 2015-02-19 3:53 AM, Piotr Wyderski wrote:
> Joerg wrote: > >> b. Next is the lazy method but quicker to develop. Buy a little iso >> module that converts 5V or whichever crircuit supply you have (or your >> 17V) and make a stable isolated 12V on the high side. Look over your >> shoulder to make sure nobody sees you doing this, hardcore analog guys >> can take someone's man card away for that. Place a sturdy gate driver >> with Schmitt input up there, Micrel or something. Then add an >> optocoupler to ferry your command signal across. That's pretty much it, >> should still be under $5, just not as elegant as method a. >> > > b'. An isolated, low mA-range, power supply of any kind and Si826x. > The part is cheap, easily available, comes in a much better package > than similar ADI parts, has an UVLO and a strong gate driver. >
Those are nice. Just keep in mind that they have a lowish UVLO. Also, they seem not to be widely stocked but for a one-off case that's ok.
> I didn't like this option because of the need for a separate > supply, but this thread has really taught me something new.
That extra supply isn't nice but as John Wayne said, "Man has got to do what man has got to do". It doesn't have to be regulated so even a simple gate drive transformer could be pressed into service to make an isolated supply.
> Thank you very much!
As a SW guy once put it, we are all here to serve :-) -- Regards, Joerg http://www.analogconsultants.com/
Reply by Winfield Hill February 20, 20152015-02-20
Winfield Hill wrote...
> > Piotr Wyderski wrote... >> >> Joerg wrote: >> >>> We know it's 17V and 50A but we'd also have to know >>> which FET you are using. That info is (hopefully) >>> in the datasheet :-) >> >> I haven't bought the FETs yet, but was thinking about >> IRFB7430PBF in TO220 or something bulkier in TO247. >> >> I naively thought that the 1524A max pulsed drain current >> is enough to survive even a short-circuit if disconnected >> in the ms range... >> >> Best regards, Piotr > > Whoa, it looks like there's a misunderstanding here. > If you short a strong low-resistance voltage source, > you actually want less current flowing, not more. > > Let's do the math. The 1524A rating of the '7430 is > for the case of a very low voltage across the MOSFET, > fully turned on, with only a volt or two of drop. > In fact, as the die rapidly heats, it'll current > limit at around 1kA, Fig 4, assuming 10V gate drive. > Trying to short out 17V, it would dissipate 17 kW. > > Z_thJC = dT_J(max) / Pd = 150/17kW = 0.009 &deg;C/W. > > Fig 14, Effective Transient Thermal Impedance, says > maximum allowed square single-pulse duration = 20us. > > That's the thermal mass calculation for the '7430. > > Let's try a higher voltage rated MOSFET (large-die > HV parts will limit at lower currents), say the > original IRF1407 version, an IRF1407PbF. It has an > impressive 333W dissipation rating, nearly as good > as the 375W rating of the '7430. But its 175&deg;C > current limit will be about 400A, or 6.8 kW. We > get Z_thJC = 150/7kW = 0.021 &deg;C/W, and datasheet > Fig 11 tells us the single-pulse duration = 35us. > So the "weaker" part can handle a full short for > almost twice as long.
To review, in a robust application, you want fast switching time, well under say 5us, and under 1us even better. This is so you can turn the switch on and check to see if it's at its proper low Vds(on) voltage. If it's not, then you have a load fault, a short, and you want to turn it off. Preferably you want to do all this within say 10us, see the 20 and 35us junction-temp limits calculated above. There are driver ICs conveniently designed for this purpose. We discuss this in AoE III, e.g., in the Logic Interfacing chapter, page 847, Figure 12.87, which shows an opto-coupled driver IC with a DESAT function. This is an industry-standard method for dealing with short-circuit faults in a robust way. In Figure 12.87 we suggest Avago's HCPL-316J and ACPL-332J. Both of these driver ICs include an optically-isolated FAULT feedback path, which your driving logic can use to deal with the fault. One way to deal with this would be to retry the power switch after a short delay to let it cool, say a few seconds. If there's still a failure after a few tries, a general FAULT condition is signaled and the user can be notified to fix the problem. -- Thanks, - Win
Reply by Winfield Hill February 19, 20152015-02-19
Piotr Wyderski wrote...
> > Joerg wrote: > >> We know it's 17V and 50A but we'd also have to know >> which FET you are using. That info is (hopefully) >> in the datasheet :-) > > I haven't bought the FETs yet, but was thinking about > IRFB7430PBF in TO220 or something bulkier in TO247. > > I naively thought that the 1524A max pulsed drain current > is enough to survive even a short-circuit if disconnected > in the ms range... > > Best regards, Piotr
Whoa, it looks like there's a misunderstanding here. If you short a strong low-resistance voltage source, you actually want less current flowing, not more. Let's do the math. The 1524A rating of the '7430 is for the case of a very low voltage across the MOSFET, fully turned on, with only a volt or two of drop. In fact, as the die rapidly heats, it'll current limit at around 1kA, Fig 4, assuming 10V gate drive. Trying to short out 17V, it would dissipate 17 kW. Z_thJC = dT_J(max) / Pd = 150/17kW = 0.009 &deg;C/W. Fig 14, Effective Transient Thermal Impedance, says maximum allowed square single-pulse duration = 20us. That's the thermal mass calculation for the '7430. Let's try a higher voltage rated MOSFET (large-die HV parts will limit at lower currents), say the original IRF1407 version, an IRF1407PbF. It has an impressive 333W dissipation rating, nearly as good as the 375W rating of the '7430. But its 175&deg;C current limit will be about 400A, or 6.8 kW. We get Z_thJC = 150/7kW = 0.021 &deg;C/W, and datasheet Fig 11 tells us the single-pulse duration = 35us. So the "weaker" part can handle a full short for almost twice as long. -- Thanks, - Win
Reply by Piotr Wyderski February 19, 20152015-02-19
Joerg wrote:

> b. Next is the lazy method but quicker to develop. Buy a little iso > module that converts 5V or whichever crircuit supply you have (or your > 17V) and make a stable isolated 12V on the high side. Look over your > shoulder to make sure nobody sees you doing this, hardcore analog guys > can take someone's man card away for that. Place a sturdy gate driver > with Schmitt input up there, Micrel or something. Then add an > optocoupler to ferry your command signal across. That's pretty much it, > should still be under $5, just not as elegant as method a. >
b'. An isolated, low mA-range, power supply of any kind and Si826x. The part is cheap, easily available, comes in a much better package than similar ADI parts, has an UVLO and a strong gate driver. I didn't like this option because of the need for a separate supply, but this thread has really taught me something new. Thank you very much! Best regards, Piotr
Reply by Winfield Hill February 18, 20152015-02-18
Piotr Wyderski wrote...
> > PS. I use the "open==conducting" meaning, if it > should be the other way around, I'll adapt.
Well, that's pretty unconventional, SFAICT. If you really do need large-die n-channel parts (consider IGBTs intead?), there are multiple ways to get reliable gate-drive power, as have been spelled out, but also including an attractive optical power source, see AoE-III, Figure 3.107, with 0.5A gate-drive for high-speed switching. Table 3.5 has these isolated switches working to 1800A pulsed, or 100A continuous. But beasts like this are likely ridiculously over-spec'd for your application. -- Thanks, - Win